nuget -install & package validation

**Usage**:

 - `scriptcs -install` - installs (with dependencies) from `packages.config`
 - `scriptcs -install {packagename}` - installs single package

**Validation**:

 - if no `packages.config` present - on start we load all .NET 4+ packages from /packages
 - if `packages.config` present - on start we recursively validate all packages from there & dependencies, and if some are missing, we notify user and don't start

**Package sources**:

 - by default we look into `appdata` for `nuget.config` (same as VS). If it's there, use package feeds from there to get packages
 - if there is a `nuget.config` inside the current folder, it takes precedence
 - if no `nuget.config` in either `appdata` or current folder we use `https://nuget.org/api/v2/`